It is well known to utilize a hydraulic tappet so that noise generated by valve mechanism can be reduced. Such a hydraulic tappet has an outer case which is in contact with a cam rotated in synchronism with a crankshaft of the engine. The outer case has an inner case, slidably and sealingly engaged therein, which actuates a valve of the internal combustion engine via rocker arm. The outer case also has a pressure chamber formed therewithin. A check valve is disposed inside or outside the pressure chamber and oil under pressure is supplied into the pressure chamber through the check valve. Accordingly, when the oil under pressure is supplied into the pressure chamber, the outer case is urged outwardly and is in contact with the cam, and the inner case is in contact with the rocker arm. When a force is applied to the outer surface from the outside thereof, due to the actuation of the check valve, the supply of the oil under pressure is stopped, and then, the hydraulic tappet serves to transmit the movement of the cam to the rocker arm.
If an orifice of a small diameter is opened to the pressure chamber, the oil under pressure flows out through the orifice. Since the amount of the flow of the oil is proportional to the time interval which is required for one cycle of the cam, the amount of the oil flow at a low rotational speed of the engine is large because the time interval for one cycle is long. As a result, the hydraulic tappet is contracted a large amount, and the lift of the valve is reduced a large amount. On the other hand, at a high rotational speed of the engine, the amount of the lift reduction of the valve is small. As mentioned above, the valve lift of the engine can be varied in accordance with changes in rotational speed of the internal combustion engine.
However, the above-mentioned mechanism can vary the valve lift of an internal combustion engine in accordance with only changes in the rotational speed of an internal combustion engine and the valve lift of the engine cannot be varied in accordance with other factors. As a result, the desired operating characteristics of the engine cannot be obtained by varying the valve lift in accordance with changes in predetermined factors, except for the rotational speed, of the engine.
To overcome the above mentioned problem, U.S. Pat. No. 3,439,661 discloses a controlled displacement hydraulic lifter, wherein the pressure chamber of the hydraulic tappet is communicated with a sliding spool control valve via an additional check valve, so that the lifting action of the lifter is controlled by the sliding spool control valve. However, in this lifter, the spool control valve is not controlled in conjunction with changes in operating conditions of the engine, such as the engine load and the temperature of the engine.